Mercury vapor lighting system and control means therefor



1, 1956 J. c. YAEGER 2,760,116

MERCURY VAPOR LIGHTING SYSTEM AND CONTROL MEANS THEREFOR Filed April 6, 1955 4 Sheets-Sheet l kn INVENTOR. JOSEPH C. Yaaean ATTORNEY Aug. 21, 1956 J. c. YAEGER MERCURY VAPOR LIGHTING SYSTEM AND CONTROL MEANS THEREFOR Filed April 6, 1955.

4 Sheets-Sheet 2 INVENTOR. .JosEPH C.Yneeea A-r-ror-mav 1956 J. c. YAEGER 2,760,116

MERCURY VAPOR LIGHTING SYSTEM AND CONTROL MEANS THEREFOR Filed April 6, 1955 4 Sheets-Sheet 3 OH. SwvrCH r t J i m i INVENTOR. JOSEPH C. mac-sea BY w fiW W A-r'roRNaY Aug. 21, 1956 c. YAEGER 2,760,116

MERCURY VAPOR LIGHTING SYSTEM AND CONTROL MEANS THEREFOR Filed April 6, 1955 4 Sheets-Sheet 4 CURRENT TRANSFORMER CONSTANT OH. QQ SWITCH P INVENTOR.

JOSEPH C.YAE.C=ER

zzdgt k A-r-roau'av United States Patent MERCURY VAPOR LIGHTING SYSTEM AND CONTRQL MEANS THEREFOR Joseph C. Yaeger, Milwaukee, Wis., assignor to McGraw Electric Company, Milwaukee, Wis, a corporation of Delaware Application April 6, 1955, Serial No. 499,548

8 Claims. (Cl. 315 122) This invention relates to a series system of mercury vapor lighting and is particularly directed to the means for controlling such a system.

In series mercury vapor lighting systems it is the usual practice to provide a plurality of mercury vapor lamps connected in series and supplied from a constant current source with each lamp bridged or shunted by a cutout such as a film cutout arranged to puncture in the event the'lamp fails, so that the remainder of the circuit is kept intact and all lamps except that bridged by the cutout stay lighted.

Mercury vapor lamps are easily started when they are cold with a fairly low voltage, but when heated they cannot be immediately restarted as the striking or starting voltage is excessively high and will often damage the lamp by flashing over or, at the least, puncturing the film cutout.

Attempts have been made to overcome the above noted defects but these attempts have not always been wholly successful. In certain improved systems time delay means have been provided which come into play when the control switch is closed to thus provide a delay to insure cooling of the lamps before voltage is applied thereto for restarting.

Other difiiculties are encountered in series systems of mercury vapor lamps, among which may be mentioned the use of a high voltage protective :relay which is connected directly in the high voltage series circuit.

This invention is designed to overcome the above noted defects and objects of this invention are to provide a series system of mercury vapor lamps and the protecting control means, therefore, which is so arranged that the high voltage protective relay usually employed in such systems is eliminated and no high voltage is brought into any portion of the control apparatus.

A further object is to provide a system of mercury vapor lighting and the protecting control means therefore, in which the lamps are immediately turned on when the control switch is closed, the delay or cooling period for the lamps being provided immediately after the lamps are extinguished.

Further objects are to provide a system of mercury vapor lighting in which the control means is a unitary structure or panel, and in which relatively simple means are employed, and in which a very much simplified sys tem of connecting the control panel with other portions of the system is secured.

Embodiment of the invention are shown in the accompanying drawings in which:

Figure 1 shows one form of the invention.

Figure 2 shows a second form of the invention in which two time delay means are provided and in which one of the time delay means is obtained through thermally responsive means.

Figure 3 shows a third form of the invention in which two time delay contactor means are employed.

Figure 4 shows a fourth form of the invention in which motor driven time delay means are employed.

Referring to Figure 1 it will be seen that a series loop "ice of mercury vapor lamps has been indicated generally by the reference character 1, the individual mercury vapor lamps by the reference character 2, and the puncturab'le film cutouts or other form of cutouts by the reference character 3. These film cutouts are bridged around each lamp so that if a lamp fails the film cutout will be punctured and the remaining lamps will remain lighted.

The series system of mercury vapor lamps is supplied through a constant current transformer or other constant current means indicated by the reference character 4. A preferred form is a static type of constant current transformer although other forms can be used.

The high tension supply lines are indicated by the reference character 5 and are connected through a main contactor or oil switch indicated generally by the reference character 6 and thence to the constant current transformer. The constant current transformer has as a unitary part a small current transformer 7 whose purpose will appear as the description proceeds.

The main contactor or oil switch 6 has the movable and stationary contacts 8 and 9, respectively, which are operated by means of the magnetic plunger 10 whose control coil is indicated by the reference character 11. This contactor is biased towards open position as shown in the drawing.

A step-down transformer indicated generally by the reference character 12 has its primary supplied from the high tension mains 5 and its secondary connected to a control unit or control panel indicated by the reference character 18. The control panel is thus supplied with low voltage from the step-down transformer 12 and the low voltage circuit is controlled by the control switch 14.

A contactor indicated generally by the reference character 15 is directly controlled by the control switch 14 and it will be seen that its operating coil 16 is connected through the stationary contacts 17 and the movable contact 13 of the contactor l9 and through the stationary contacts 2i) and the movable contact 21 of the contactor 22. As shown in Figure 1, closing of the contactor 15 establishes a holding circuit through the resistor 23 and the contacts 17 and 18 of the contactor 19. When the contactor 15 closes it establishes a circuit for the operating coil 24 of the contactor 22 and the contactor 22 opens. Closing of the contactor 15 also closes the circuit of the operating coil 11 of the main contactor 6 for the mercury vapor lamp circuit. It is to be noted that reclosing of the contactor 22 is delayed by means of a delay means 25 which does not delay opening of the contactor but does delay its closing for a greater length of time than the cooling time of the mercury vapor lamps as will be described in greater detail hereinafter.

A fourth contactor indicated generally by the reference character 26 is provided for the control panel and its operating coil 27 is permanently connected to the current transformer or overload transformer '7 which, as stated, forms a unitary part of the constant current transformer t. The contactor 26 is biased open and remains in open position under normal load conditions. When a overload occurs, as when there is a breakdown in the constant current transformer, or when there is an open circuit in the mercury vapor system, the contactor 26 closes. This causes energization of the operating coil 17 of the contactor 26 sufficient to overcome the bias of the contactor 26 and causes it to close. Closing of the contactor 26 causes energization of the operating coil 28 of the contactor l9 and opens the control circuit of the contactor 15 and causes such latter contactor to drop open. Energization of the contactor 19 also causes deenergization of the contactor 22 and such latter contactor tends to slowly close being delayed in its closing motion by the time delay means 25, it being noted that the contactor 22 is biased towards closed position. Energization of the contactor 19 also establishes a holding circuit for itself as its other movable contact 29 bridges the stationary contacts 30 and therefore contactor 19 opens the circuit for the control coil 16 of contactor 15 and holds such circuit open. When the contactor 15 is deenergized and drops open it deenergizes the main contactor 6 and contactor 6 opens the high tension circuit to the mercury vapor lamps.

Contactor 19 will remain energized and, as described hereinabove, main contactor 6 will remain open. It is necessary to open control switch 14 in order to deenergize contactor I9. Thereafter control switch 14' may be closed provided the defect above noted has been corrected, and the high tension supply source will then be operatively connected to the mercury vapor lamps through the operation of the control unit or control panel 13 as previously described.

The control unit or control panel also takes care of other abnormal conditions. For example, in the event of power failure contactors 15 and 6 will move to their positions as shown in Figure l. Contactor 22 will start moving to closed position being delayed in such action by its time delay means 25. In the event that power is restored in less time than that required for the cooling of the mercury vapor lamps the circuit for the mercury vapor lamps will remain open until contactor 22 has finally closed thus giving ample cooling time for the mercury vapor lamps.

Another important characteristic of the control unit is that contactor 15 is responsive to low voltage conditions when the voltage drops below a predetermined value since its holding circuit is established through the resistor 23. Under low voltage conditions there is not enough current flow to maintain contactor 15 in its closed position and such contactor drops open thus opening the control circuits of the contactor 22 and main contactor 6. Restoration of normal voltage conditions will not cause energization of the mercury vapor lamps until the cooling period has passed due to the time delay of contactor 22.

The disclosure described hereinabove for a static type constant current transformer will be changed in the event that a floating coil type of transformer is used in place of a static type. in the event a floating coil constant current transformer is used the current transformer 7 is discarded and a step-down potential transformer is bridged across the secondary or output terminals of the floating coil transformer and the secondary of the potential transformer is connected in series with the control coil 27 of the contactor 26. Another way of using a floating coil type of transformer would be to use a current transformer for the series circuit of the mercury vapor lamps, with r its secondary connected to the energizing coil 27 of the contactor 26 but in this case the contactor 26 would be biased closed and would be held open as long as the series circuit of the mercury vapor lamps remain intact.

Any of the above means may be employed in order to control contactor 26 so that contactor 26 will respond to abnormal conditions in the high tension or high voltage circuit, the current transformer 7 being intended to indicate any desired means to accomplish such purpose.

In any of the different forms of the invention hereinafter described, it is to be understood that any of the substitutions for the current transformer 7 and the associated parts could be made as hereinabove described.

In the form of the invention shown in Figure 2 it will be seen that the main contactor 6, the constant current transformer 7, the series loop 1 of mercury vapor lamps, the contactor 26, and the contactor 19 are the same as that described in connection with Figure l.

A contactor 31 has its lower movable contact 32 arranged to bridge the stationary contacts 33 and energize the control coil 11 of the main contactor 6 and thus control the operative connection of the series system of mercury vapor lamps to the high voltage source. Contactor 31 also establishes a holding circuit for itself by means of its upper movable contact 34 which bridges its station- 4,. ary contacts 35 and thus establishes the holding circuit through the resistor 36 and the movable and stationary contacts 18 and 17 of contactor 19.

An additional contactor indicated generally at 37 is provided and has an operating coil 38 and time delay means 39 which delays its closing for a very brief period of the order for example of 45 seconds, though this number is not to be considered as limiting but merely as illustrative. C-ontactor 37 closes upon closing of the control switch 14 after the brief delay period mentioned above. The purpose of the contactor 37, which is delayed in closing for the brief period hereinabove mentioned but which is not delayed in its opening motion, is to guard against the effect of transients due to disturbances in the line from switching or other causes. It serves to delay the closing of contactor 31 for the brief period mentioned so as to allow time for the subsiding of these transients.

A new type of delay means different from that hereinbefore described in connection with Figure 1 is provided in the form of a thermally responsive unit. This unit consists of a bimetal strip or member 40 fixed at one end it and biased closed so that its movable contact 42 at its free end normally engages its stationary contact 43 when the bimetal member is cold. The bimetal member 41 takes about 7 minutes or more to cool and to thus close its contacts after it has been once heated. The seven minute delay is merely given as illustrative. The delay period in closing is greater than the cooling time of the mercury vapor lamps and may be of any duration depending on the type of mercury vapor lamp employed provided always that it does not close its contacts until a sufiicient time has elapsed to allow cooling of the mercury vapor lamps. The bimetal member or thermal member is heated by means of the heater 44 whose circuit is completed as soon as contactor 31 closes.

It is to be noted that all of the control elements are carried as a unitary structure and constitute a control unit or control panel indicated generally by the reference character 45. It is to be noted also that no high voltage circuits are brought into the control unit. Further it will be seen that the contactor 26 is controlled in identically the same manner as previously described.

The operation of the apparatus shown in Figure 2 is as follows. When the control switch 14 is closed, it is apparent that a circuit is established through the bimetal 40 to the control coil 38 of contactor 37. After a slight delay, as hereinabove described, contactor 37 closes and energizes the control coil 46 of contactor 31 and thus causes closing of contactor 31 and closing of the main contactor 6. Closing of the contactor 31 establishes a holding circuit for itself through the resistor 36 and the stationary and movable contacts 17 and 38 of contactor 19. The contactor 31 is therefore responsive to under voltage for when the voltage drops below a predetermined value the reduced current flow in the coil 46 of contactor 31 due to the presence of the resistor 36 will cause con tactor 31 to drop open and cause main contactor 6 to open. This also cuts off the supply of current to the heater 44 and allows the thermal element to cool and to close its contacts 42 and 43 after a lapse of time greater than the cooling period of the mercury vapor lamps. Obviously if normal voltage is again established and if the thermal element 40 has had sufficient time to close its contacts, contactor 31 will again close after the slight delay provided by contactor 37 and will thus cause closing of the main contactor 6.

If contactor 26 should close for any of the reasons hereinbefore enumerated, it is apparent that contactor 19 will be energized and will open the control circuit of contactor 3]. and at the same time will establish a holding circuit for itself. This, of course, causes opening of main contactor 6. Main contactor 6 will remain open until the control switch 14 has been opened and reclosed as previously described in connection with Figure 1.

In the form of the invention shown in Figure 3 very much of the same scheme as that shown in Figure 1 is employed. Contactors 6, 15, 19, 22 and 27 are exactly as described in connection with Figure 1. However, a new contactor indicated generally by the reference character 47 has been added and its control coil 48 is directly energized when the control switch 14 1s closed. This contactor 47 has time delay means 49 which delays its closing for a very brief period but which does not delay its opening. Contactor 47 directly controls the closing of contactor after the lapse of a brief period hereinabove mentioned. This brief period may be of the order of 45 seconds though this exact interval is not to be understood as limiting but merely as an example. Contactor 47 closes upon closing of the control switch 14 after the lapse of the brief period mentioned above. The slight delay in closing is to guard against the efiect of transients due to disturbances in the line from switching or other causes.

After contactor 47 closes the system functions in exactly the same manner as that described in connection with Figure l.

In the form of the invention shown in Figure 4 contactors 6, H, and 26 are identical with those previously described and similarly numbered. In this form of the invention contactors 50 and 51 are provided and have control coils S2 and 53 respectively. This form of the invention also has motor driven time delay means indicated generally by the reference character 54 and consisting of a motor 55 which drives a cam 56 through a reducition gear means, and a cam follower or switch member 57 having contacts 58 and 59. The control panel or unit is indicated generally by the reference character When the control switch 14 is open all of the parts are in the position shown in Figure 4.

When the control switch 14 is closed contactor 50 closes and closes the motor circuit through movable contact 61 and stationary contacts 62. Also contactor 50 closes the circuit for the control coil 53 of contactor 51 through the medium of movable contact 63 and stationary contact 64. Contactor 5'1 closes and establishes a holding circuit for itself through resistor 65 and stationary and movable contacts 17 and 18 of contactor 19. In addition to this contactor 51 closes the circuit of control coil 11 of main contactor 6 through the medium of movable contact 65 and stationary contacts 66. Contactor 51 also withdraws its movable contact 67 from its stationary contacts 68.

The motor driven time delay means 54 operates the cam 56 and opens contacts 58 thus deenergizing contactor 5% which drops to open position. However, contactor 51 remains closed due to its holding circuit. When the cam follower or switch arm 57 of the motor driven time delay means 5 engages the high side of the cam 56 it closes contact 5'9 and opens contact 58. The motor, however, does not continue to operate since movable contacts 67 has been withdrawn from stationary contact 68 of contactor 51.

In the event the voltage should drop below a predetermined value the contactor 51 would drop to the position shown in Figure 4 as its holding circuit including the resistor 65 does not allow suflicient current in the energizing coil S3 to hold contactor 51 in closed position. When this occurs the motor circuit is completed through contacts 59 of the time delay means and contacts 67 and 63 of contactor 51. The motor continues to drive the cam 5d until the cam follower or switch arm 57 engages the low side of the cam. This provides the necessary time delay to allow cooling of the lamps.

in the event of a break in the series lamp circuit or overload or breakdown of the constant current transformer the control unit or control panel will operate in the manner hereinbe'fore described.

It is to be noted that contactors 15 of Figure 1, 31 of Figure 2, 15 of Figure 3, and 51 of Figure 4 will all drop out under low voltage conditions when the voltage drops below a predetermined value. The characteristic of these contactors is such that they will not pull in even though their control coils may be directly connected across the low voltage control circuit and though the resistor is not included in this circuit, unless the voltage conditions closely approximate normal voltage.

it will be seen that in each form of the invention a main or high voltage contactor is employed for controlling the high voltage lamp circuit. This contactor is indicated by the reference character 6 in each of the views.

Further it is to be noted that in the control unit itself there is a primary contactor for example the contactors 15 in Figures 1 and 3 and the contactors 41 and 51 in Figures 2 and 4. Each control unit is provided with overload contactor means which may include the overload lock-in contactor 19 in each of these views and the overload contactor proper 26 in each of the views. The specific purpose of employing two contactors for the overload contactor means is so that the low voltage circuit or panel supply circuit can be used for energizing the holding circuit of one of these two contactors. It is to be noted further that in each form of the invention time delay contactor means is employed as shown for instance by the contactors 22 of Figures 1 and 3 or the thermal contactor of Figure 2 or the contactor in conjunction with the time delay means 54 of Figure 4. These last mentioned contactors provide the time delay for the closing period of the high voltage circuit after the lamps have been once lighted and subsequently extinguished, to thereby provide the necessary delay to insure cooling of the lamps before the application of high voltage thereto. In addition to this, Figure 2 shows a form of the invention in which an auxiliary or short time delay contactor 37 is employed to delay the closing of the lamp circuit for a brief period to allow for the subsiding of any transients.

it is to be understood that the term contactor is not intended as a limiting expression, but is intended to cover contactors, relays, circuit interrupters, or other devices of this general nature. Further the expression constant current transformer is intended to cover not only constant current transformers but also constant current devices of any suitable nature.

It will be seen that a relatively simple control panel or control unit has been provided in which no portion of the high voltage circuit passes through or into such control unit.

It will be seen further that the lamps cannot be restarted after once having been lighted until they have had suflicient time to cool down, and that this time delay is pr0- vided during the time when the lamps are not lighted thus allowing the lamps to immediately come on when the control switch is closed.

It is to be understood that the control switch may be manually or automatically operated or may be of any other suitable control switch construction.

Although this invention has been described in considerable detail, it is to be understood that such description is intended as illustrative rather than limiting, as the invention may be variously embodied and is to be interpreted as claimed.

I claim:

1. A low voltage control unit for controlling a high voltage series circuit of mercury vapor lamps supplied from a high voltage source and provided with a main contactor having a control coil, a first step-down transformer connected to said high voltage source for supplying low voltage to said control unit, and a second stepdown transformer having its primary operatively connected to said series circuit and having a secondary for supplying low voltage secondary current whose value responds to the condition of said series circuit; said low voltage control unit comprising a low voltage supply circuit supplied from said first transformer and provided with a control switch, a primary contactor having a control coil for energization from said low voltage circuit, time delay means having normally closed contacts connected in the circuit of the control coil of said primary contactor, said time delay means being energized from said primary contactor to open its contacts when said control switch is closed and said primary contactor is energized and having delay means for delaying the closing of its contacts, and low voltage contactor means having an energizing coil connected to said second stepdoWn transformer and having contacts connected in the circuit of the control coil of said first contactor, whereby the control unit is isolated from the high voltage of the series circuit and whereby said first contactor controls said main contactor in response to the condition of the series circuit.

2. A series system of mercury vapor lighting comprising a plurality of mercury vapor lamps connected in a series circuit, a cutout shunted across each lamp for shorting out any lamp upon failure thereof, a source of high voltage connected to said series circuit, a main contactor controlling the connection of said source to said series circuit, a normally open overload contactor having a control coil, a transformer having its primary connected to said series circuit and having a low voltage secondary connected to the control coil of the overload contactor, a normally closed lock-in overload contactor having a control coil and a holding circuit, said holding circuit being closed when said lock-in overload contactor is open, said overload contactor controlling the energization of the control coil of said lock-in overload contactor, a source of low voltage for said lock-in overload contactor, a primary contactor controlling said main contactor and being normally open, said primary contactor having a control coil connected to the source of low voltage through said normally closed lock-in overload contactor; and means including a control switch for controlling the energization of the primary contactor.

3. A series system of mercury vapor lighting comprising a plurality of mercury vapor lamps connected in a series circuit, a cutout shunted across each lamp for shorting out any lamp upon failure thereof, a source of high voltage connected 'to said series circuit, a main contactor controlling the connection of said source to said series circuit, a transformer having its primary connected to said series circuit and having a low voltage secondary; a control unit comprising a normally open overload contactor having a control coil connected to the secondary of said transformer, a normally closed lock-in contactor having a control coil and a holding circuit, said holding circuit being closed when said lock-in contactor is open, said overload contactor controlling the energization of the control coil of said lock-in contactor, a source of low voltage for said lock-in contactor, a primary contactor controlling said main contactor and being normally open, said primary contactor having a control coil connected to the source of low voltage through said normally closed lock-in contactor; and means including a control switch for controlling the energization of the primary contactor.

4. A series system of mercury vapor lighting comprising a plurality of mercury vapor lamps connected in a series circuit, a cutout shunted across each lamp for shorting out any lamp upon failure thereof, a source of high voltage connected to said series circuit, a main contactor controlling the connection of said source to said series circuit, a transformer having its primary connected to said series circuit and having a low voltage secondary, a normally open overload contactor having a control coil connected to the secondary of said transformer, a normally closed lock-in contactor having a control coil and a holding circuit, said holding'circuit being closed when said lock-in contactor is open, said overload contactor controlling the energization of the control coil of said all lock-in contactor, a source of low voltage for said lock-in contactor, a primary contactor controlling said main contactor and being normally open, said primary contactor having a control coil connected to the source of low voltage through said normally closed lock-in contactor, time delay means including contact means delayed in their closing motion and connected in the circuit of the control coil of the primary contactor, and means including a control switch for controlling the energization of the primary contactor.

5. A series system of mercury vapor lighting comprising a plurality of mercury vapor lamps connected in a series circuit, a cutout shunted across each lamp for shorting out any lamp upon failure thereof, a source of high voltage connected to said series circuit, a main contactor controlling the connection of said source to said series circuit, a transformer having its primary connected to said series circuit and having a low voltage secondary, a normally open overload contactor having a control coil connected to the secondary of said transformer, a normally closed lock-in contactor having a control coil and a holding circuit, said holding circuit being closed when said lock-in contactor is open, said overload contactor controlling the energization of the control coil of said lock-in contactor, a source of low voltage for said lock-in contactor. a primary contactor controlling said main contactor and being normally open, said primary contactor havii control coil connected to the source of low voltage through said normally closed loch-in contactor, time delay contactor means closed when deencrgized and opened when energized and controlled from said primary contactor and having tii we delay means for delaying its closing after said primary contactor has opened, and means including a control switch for controlling the energization of the primary contactor.

6. A series system of mercury vapor lighting comprising a plurality of mercury vapor lamps connected in a series circuit, a cutout shunted across each lamp for shorting out any lamp upon failure thereof, a source of high voltage connected to said series circuit, a main contactor controlling the connection of said source to said series circuit, a transformer having its primary connected to said series circuit and having a low voltage secondary, a normally open overload contactor having a control coil connected to the secondary of said transformer, 21 nor mally closed lock-in contactor having a control coil and a holding circuit, said holding circuit being closed when said lock-in contactor is open, said overload contactor controlling the energization of the control coil of said lock-in contactor, a source of low voltage for said lock-in contactor, a primary contactor controlling said main contactor and being normally open, said primary contactor having a control coil connected to the source of low voltage through said normally closed lock-in contactor. time delay contactor means closed when deenergized and opened when energized and controlled from said primary contactor and having time delay means for delaying its closing after said primary contactor ins opened, a short time delay contactor means open when dccnergized and closed when energized and including time delay means delaying its closing motion, said short time delay contactor means delaying the closing of s d primary contactor, and means including a control switch for controlling the energization of the primary contactor.

7. A series system of mercury vapor lighting comprising a plurality of mercury vapor lamps connected in a series circuit, a cutout shunted across each 1 p for shorting out any lump upon failure thereof, a so Joe of high voltage connected to said series circuit, main contactor controlling the connection of said source to said series circuit, a primary contactor controlling said train contactor and being normally open, said primary contactor having a control coil, a source of low voltage, a time delay contactor delayed in its closing motion and energized for opening motion upon closing of the primary contactor, a second normally open time delay contactor delayed in its closing motion, both of said time delay contactors having their contacts connected in series with the energizing coil of said primary contactor and controlling the connection of said controlling coil with the source of low voltage, and a control switch controlling the energization of the contactor Whose closing is delayed.

8. A series system of mercury vapor lighting comprising a plurality of mercury vapor lamps connected in a series circuit, a cutout shunted across each lamp for shorting out any lamp upon failure thereof, a source of high voltage connected to said series circuit, a main oontactor controlling the connection of said source to said series circuit, a primary contactor controlling said main contactor and being normally open, said primary contactor having a control coil, a source of low voltage, a

10 time delay contactor delayed in its closing motion and energized for opening motion upon closing of the primary contactor, a second normally open time delay contactor delayed in its closing motion, both of said time delay contactors having their contacts connected in series with the energizing coil of said primary contactor and controlling the connection of said controlling coil with the source of low voltage, a control switch controlling the energization of the contactor Whose closing is delayed, a transformer having its primary connected to said series circuit and having a 'low voltage secondary, and overload contactor means controlled from the secondary of said transformer for opening the circuit of the control coil of said primary contactor.

No references cited. 

